(1) Field Of The Invention
The invention relates to a self-supporting sheet-like article comprising a substrate layer and an antistatic coating on at least one surface of the substrate layer.
The invention also relates to a process for the production of a sheet-like article according to this invention with an antistatic coating and to a firmly adhering antistatic coating on a polymer substrate.
(2) Prior Art
Because of their chemical composition, many working materials are not at all conductive for electric current or only conductive to a degree which practically cannot be measured, and therefore they are generally considered to be insulators. However, the low conductivity of these working materials is the reason that they often collect very high electrostatic charges at their surfaces which, especially with sheet-like polymer products such as films and panels, can lead to a multitude of disadvantageous disturbances, even to a complete uselessness of the product. This is highly undesirable. The charges occur during production, processing, or use of the sheet-like products, wherever there are friction processes at the edges, turning rods, etc., where two materials touch and then separate again (separation process), for example, at rollers. This can lead to production disturbances, dust collection, interferences with measuring and control instruments, and even to explosions. When sheet-like polymer substrates are printed, i.e. using screen-printing processes, the electrostatic charges can have such a negative effect that the ink "jumps" from the printing form or stencil. High static charges (more than 5,000V) can ignite the gas/air mixtures of the evaporated solvents, and low static charges (less than 1,000V) can optically affect or destroy the surfaces by causing the formation of strias and dust and dirt collection.
The scientific and physical relationships which lead to the occurrence of electrostatic charges are not always clear. However, generally it is assumed that, upon contact between solid bodies, electrons or ions change from one body to another. This change is accelerated by friction because the actual contact areas become larger.
The amount of the electrostatic charge depends on the electrical conductivity of the materials. Good conductors are, for example, metal, carbonblack, graphite, and polyenes. As good conductors, they loose their charge immediately, while bad conductors, especially polymers, maintain their charge over a longer period of time (seconds or minutes and sometimes even days).
The aforementioned problems as well as several attempts to solve them have been described and discussed in numerous references, for example, B. Naber, "Anti-Static Equipment of Polymers, A Literature Review" in "Plaste und Kautschuk" (1981), pages 6-9. However, all the many conventional solutions need improvement, especially with regard to the uniformity and durability of the antistatic equipment.
Therefore, it was an object of this invention to develop sheet-like articles such as films, panels, fabrics, particularly webs, of materials with low electrical conductivity, which have superior antistatic characteristics, especially with regard to the uniformity of the antistatic coating over the entire surface as well as the resistance to mechanical effects.